1. Field
This invention relates to the automatic ultrasonic inspection of welds and other test objects. The invention particularly relates to the automatic inspection of the circumferential welds which join together sections of pipelines which transmit petroleum products and other fluids. These welds are known as girth welds. Recently automatic devices have been developed for rapidly welding together sections of pipeline. These automatic devices typically carry a welding device around the pipeline on a tack which is mounted circumferentially on the pipeline. These devices occasionally malfunction and reproduce the same type of flaw in weld after weld. For this reason it is important quickly to detect the presence of an unacceptable flaw in such a weld and to correct the malfunction before additional defective welds are produced. Detecting flawed welds before the pipeline is buried is important because unearthing buried pipeline is very expensive and because a pipeline explosion can destroy property and take human lives.
2. Prior Art
Inspecting welds and other test objects by devices which move along the welds, transmit ultrasonic waves into the welds and detect ultrasonic waves reflected back from the welds is not new. See, for example, U.S. Pat. No. 2,953,017 (1960) to Bincer, et al., U.S. Pat. No. 3,068,370 (1962) to McInnish, U.S. Pat. No. 3,213,676 (1965) to Makous, U.S. Pat. No. 3,248,933 (1966) to Stebbins, U.S. Pat. No. 3,575,044 (1971) to Gibbs and U.S. Pat. No. 3,712,119 (1973) to Cross. As used in this description and in the claims, the term "weld" includes not only the filler, or the metal added, but also the adjacent heat-affected zones of the metal joined by the filler. As used herein, the term "reinforcement" of a weld shall mean the cap on the external surface of the weld or the stringer bead on the internal surface of the weld. Typically these reinforcements are convex accumulations of metal at the upper and lower boundaries of the weld and reflect ultrasonic waves.
The ultrasonic transducers used in such devices are well known in the art and throughout this description and in the claims will be referred to as "ultrasonic transducers" or simply "transducers". These transducers have crystals which are electrically energized and which transmit ultrasonic waves into adjacent regions. Conversely, the crystals may receive ultrasonic waves impinging on the crystals and the transducers may convert these received waves by well known means into electrical signals for display on oscilloscope screens or other media. As used throughout this description and in the claims, these ultrasonic sound waves will be referred to as "ultrasonic waves" or simply "waves".
In the prior art, automatic ultrasonic devices for inspecting girth welds may include a track placed around the pipeline near the weld. The transducers may be carried in an open-ended housing which is mounted on a carriage guided by the track and is carried along the track adjacent the weld to inspect the weld. See, for example, U.S. Pat. No. 3,248,933 to Stebbins. Coupling liquid such as water may be interposed between the transducers and the pipeline to improve the propagation of ultrasonic waves between the transducers and the pipeline. This liquid may be retained around the transducers by a flexible seal or gasket which is mounted around the open end of the housing and which slidably engages the pipeline.
A problem unsolved in the prior art is the problem of rapidly and properly aligning the path of the transducers with respect to the weld, so that the transducers will traverse the weld with distances from the weld and angular orientations or attitudes with respect to the weld maintained substantially constant. Undesired changes in such distances and angular orientations can render the signals received from the transducers meaningless and can necessitate the continuous and time consuming reorientation and recalibration of the inspection apparatus.
In the known prior art, no satisfactory technique has been developed for keeping the loss of coupling liquid minimal and yet insuring that there is always coupling liquid between the transducers and the pipeline. Inevitably, some liquid will leak, particularly if the surface of the pipeline is rough. If this leakage becomes too great, air may intrude between the transducers and the pipeline and the propagation of ultrasonic waves will be greatly diminished or interrupted, with resulting confusion in the meaning of the signals from the transducers. The likelihood of such an interruption may be decreased by increasing the pressure of the water in the apparatus, but this step increases the amount of liquid cost during the inspection of each weld. In many kinds of terrain, great loss of such liquid, even if only water, is unacceptable.
In the known prior art, no satisfactory method has been devised for correlating accurately the indications of flaws in the weld with the position of the transducers at the time such flaws were detected in order to determine the location of the flaws. This correlation is particularly important when the transducers are traversing rapidly a lengthy weld.
When a wave transmitted from one of the transducers reaches the top or the bottom reinforcement of a weld, at least part of that wave will be reflected by the reinforcement. It is important to distinguish such reflected waves from those other waves reflected by actual flaws in the weld. In the known prior art, no satisfactory method is shown for eliminating signals associated with waves reflected by the upper and lower boundaries of the welds and received by the transducers.